JPH11212327A - Multicolor image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multicolor image forming device which improves gradation in the highlight part of an image by a simple, inexpensive manner. SOLUTION: Toner in each of at least three colors, cyan, magenta, and yellow, used in developing units 15, 16, and 17 as developing means is composed of a mixture of two or more kinds of toners Ta and Tb of different characteristics. Thus, for example, the light-colored toner Ta whose electrification amount is small and light transmittance is high starts to stick to the surface of a photoreceptor drum 9, and then the dark-colored toner Tb whose electrification amount is large and light transmittance is low stucks to the surface of the photoreceptor drum 9, so that both the toners Ta and Tb stick to the surface of the photoreceptor drum 9 as a composite toner image of a wide dynamic-range. Therefore, a latent image in the highlight part where charges on the photoreceptor drum 9 are low is developed with only the light-colored toner Ta whose electrification amount is small and light transmittance is high, and the high gradation image can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a facsimile, and a printer.
The present invention relates to a multicolor image forming apparatus that forms a color image using color toners such as yellow, magenta, and cyan.

[0002]

2. Description of the Related Art In an electrophotographic image forming apparatus, the number of reproducible tones greatly affects the image quality of an output copy. In particular, in a multicolor image forming apparatus that forms a color image by using color toners such as yellow, magenta, and cyan, there is a tendency that the gradation of a highlight portion of the formed image is hard to appear, and Deterioration of the gradation in the highlight portion is one of the causes of deterioration in the quality of the copied image. That is, the highlight portion of the color image formed by the electrophotographic method has a sharp gradation characteristic between the reflection density of the image of the transfer paper itself and the reflection density of the portion where the toner starts to adhere. Is not smooth (broad). This is one factor that makes an image obtained by an electrophotographic multicolor image forming apparatus inferior to a normal color photographic image.

[0003] Conventionally, as a method of solving such deterioration of the gradation of the highlight portion of the image, the gradation of each of the yellow, magenta, cyan and black images is adjusted so that the gradation is not lost. YMC to correct
A method of correcting the optical power output of a laser beam as a latent image forming means for image data by using a K gradation correction table (γ correction table) so as to improve visual gradation characteristics (Japanese Patent Laid-Open No. 9-107476). "Image forming apparatus" described in Japanese Patent Application Laid-Open Publication No. HEI 9-209; this is hereinafter referred to as "known example 1"). When the image density in the dither matrix is equal to or less than a certain threshold value, the image density is propagated to an adjacent matrix, thereby achieving both gradation characteristics and gradation stability of a highlight portion (low density portion) of the image. (Japanese Unexamined Patent Publication No. 6-1649)
08, "A halftone image recording method", hereinafter referred to as "known example 2"). A method of controlling the toner charge amount, the developing bias, the developing gap, and the like by mathematical expressions in order to suppress the roughness of the highlight portion of an image (“Image forming apparatus” described in JP-A-6-270442; 3 "). A method of shifting the original density-copy density characteristic by adjusting the exposure amount to form an image a plurality of times for each color to obtain characteristics close to ideal characteristics over a wide density range (Japanese Patent Publication No. 6-46329). "Electrostatic transfer type recording apparatus"; hereinafter, this is referred to as "known example 4"). And so on.

[0004]

However, in the method of the "known example 1", the process of preparing the gamma correction table is complicated, and the relationship between the image data and the light output of the laser light is calculated using the correction table. Is distorted, the number of gradations of the actual light output is reduced, and the image tends to be coarse.

In the method of "known example 2", since the image dot size is increased by the matrix size by using the dither matrix, as described in "known example 3", the so-called image There is a disadvantage that the resolution is reduced.

The method of "known example 3" can be said to be an effective method in terms of suppressing roughness of a highlight portion of an image, but the gradation of the highlight portion is not described because it is not described. .

Furthermore, the method of "known example 4" has a feature that a very wide image density dynamic range can be obtained. However, since it is necessary to perform image transfer for each color a plurality of times, one copy is required. There is a problem that the image forming time per contact becomes longer. Further, in this method, the number of times of image transfer is twice the number of times of image transfer of a general image forming apparatus.
It is necessary to perform the positioning of the transfer sheet at the time of this image transfer with extremely high accuracy, and there is a high possibility that image blurring due to the displacement of the transfer sheet at the time of this image transfer occurs.

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to form a multicolor image capable of improving the gradation of a highlight portion of an image by a simple and inexpensive method. It is to provide a device.

[0009]

In order to achieve the above object, the present invention is directed to a method for forming a latent image on an image carrier using image light modulated based on image information. Latent image forming means, and the latent image on the latent image carrier formed by the latent image forming means, at least, yellow, magenta,
Developing means for converting the latent image into a toner image using three cyan toners; and transferring the toner image visualized on the image carrier by the developing means onto a transfer member. And a fixing unit for fixing the toner image transferred on the transfer member by the transfer unit to the transfer member, the toner used in the developing unit is Each of the toners of the respective colors comprises a toner obtained by mixing two or more toners having different characteristics.

In this multicolor image forming apparatus, each of the toners of at least three colors used in the developing means is formed by mixing two or more kinds of toners having different characteristics from each other. Thereby, the latent image corresponding to one color formed in the image forming means is formed into a toner image by a plurality of toners of the same color having different characteristics from each other. The latent image can be visualized with toner having characteristics closest to the original image.

According to a second aspect of the present invention, in the multi-color image forming apparatus of the first aspect, each of the toners of the respective colors is formed by mixing a plurality of toners having different light transmittances. The higher the light transmittance of each toner, the lower the charge amount of the toner.

In this multicolor image forming apparatus, each of the toners of at least three colors used in the developing means is composed of a mixture of a plurality of toners having different light transmittances. The higher the light transmittance of each toner, the lower the charge amount of the toner. As a result, the highlight portion of the latent image is visualized with a light-colored toner having a low charge amount and a high light transmittance, and the high-density portion of the latent image has a high charge amount and a light transmittance. The toner image is visualized by a dark color toner having a low color (to be described in detail later). Therefore, in this multicolor image forming apparatus, it is possible to form an image having a wide dynamic range without performing a plurality of times of image transfer for each color as described in the above-mentioned “known example 4”. Become.

According to a third aspect of the present invention, in the multi-color image forming apparatus according to the first aspect, each of the toners of the respective colors comprises a toner in which a plurality of toners having different lightness, saturation, and hue are mixed, Further, the toner is characterized in that the higher the brightness, saturation, and hue of each mixed toner, the lower the charge amount of the toner.

In this multicolor image forming apparatus, each of the toners of at least three colors used in the developing means is a toner in which a plurality of toners having different lightness, saturation, and hue are mixed, and The higher the brightness, saturation, and hue of each mixed toner, the lower the charge amount of the toner. As a result, the highlight portion of the latent image is visualized with a toner having a low charge amount and high brightness, saturation, and hue, and the high density portion of the latent image has a high charge amount and a high brightness. , And a visible image is formed with a toner having a low color saturation and hue (the details will be described later). Therefore, in the multicolor image forming apparatus, it is possible to form an image having a wide dynamic range and a wide color reproduction range, and having improved gradation in a highlight portion.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a multicolor image forming apparatus (hereinafter, referred to as a copying machine) which is an image forming apparatus will be described. FIG. 1 is a schematic configuration diagram of a copying machine 100 according to the present embodiment, and FIG. 2 is an enlarged view around a photosensitive drum and an intermediate belt. Hereinafter, FIGS. 1 and 2
The configuration and operation of the copying machine 100 will be described with reference to FIG.
A color image reading device (hereinafter, referred to as a color scanner) 1 converts an image of a document 3 into an illumination lamp 4, a mirror group 5a.
5c and the lens 6 to form an image on the color sensor 7 to convert the color image information of the original into, for example, blue (Blue,
B), Green (hereinafter G),
It is read for each red (Red) color separation light and converted into an electrical image signal. The color sensor 7
In this example, B, G, and R color separation means and a photoelectric exchange element such as a CCD are used to read three colors simultaneously. Then, based on the color separation image signal intensity levels of B, G, and R obtained by the color scanner 1, a color conversion process is performed by an image processing unit (not shown), and black (hereinafter, referred to as Bk), cyan (Cyan, hereinafter C), magenta (Magenta, hereafter M), and yellow (Yellow, hereafter, Y) color image data are obtained. The color image recording device (hereinafter, referred to as a color printer) 2 visualizes Bk, C, M, and Y to obtain a final color copy. The operation system of the color scanner 1 for obtaining the image data of Bk, C, M, and Y is such that the illumination / mirror optical system shown in FIG. Scans the document in the direction of the left arrow, and obtains one color image data for each scan. Each time, the color printer 2 sequentially superimposes the images to form a four-color full-color image.

Next, an outline of the color printer 2 will be described. The writing optical unit 8 converts the color image data from the color scanner 1 into an optical signal, performs optical writing corresponding to the original image, and forms an electrostatic latent image on the photosensitive drum 9. The unit 8 includes a laser light source 8a, a light emission drive control unit (not shown), a polygon mirror 8b, a rotation motor 8c, an f / θ lens 8d, and a reflection mirror 8e.
And so on.

The photosensitive drum 9 rotates in a counterclockwise direction as shown by an arrow. Around the photosensitive drum 9, as shown in FIG.
Static elimination lamp 11, charging device 12, potential sensor 13, Bk developing device 14, C developing device 15, M developing device 16, Y developing device 1
7, an optical sensor 18 for detecting a development density pattern, an intermediate transfer belt 19 and the like are arranged.

Each of the developing devices includes a developing sleeve (14a, 15a, 16a, 17a) which rotates by bringing an ear of developer into contact with the surface of the photosensitive drum 9 to develop an electrostatic latent image;
It is composed of a developing paddle (14b, 15b, 16b, 17b) that rotates to pump and agitate the developer, and a toner concentration sensor (14c, 15c, 16c, 17c) of the developer. In the standby state, the agent on the developing sleeve in all four developing units is in a state of cutting off (development inoperative). Next, an outline of the copying operation will be described with reference to an example in which the order of the developing operation (color image forming order) is Bk, C, M, and Y (however, the image forming order is not limited to this).

When the copying operation is started, reading of Bk image data is started at a predetermined timing by the color scanner 1, and based on this image data, light writing by a laser beam and formation of a latent image are started (hereinafter, Bk image data). Are referred to as Bk latent images. C, M, and Y are referred to as C latent images, M latent images, and Y latent images, respectively). This B
Before the leading end of the latent image reaches the developing position of the Bk developing device 14 so that development can be performed from the leading end of the k latent image, the developing sleeve 1
4a is started to rotate and the spike of the agent is performed, and the Bk latent image is
Develop with toner. Thereafter, the developing operation of the Bk latent image area is continued, and when the trailing end of the latent image has passed the Bk developing position, the agent on the Bk developing sleeve 14a is promptly cut off to put the developing inoperative state. This is completed at least before the leading end of the C latent image based on the next C image data arrives. The ear cutting is performed by switching the rotation direction of the BK developing sleeve 14a to a direction opposite to that during the developing operation.

The Bk toner image formed on the photosensitive drum 9 is transferred onto the surface of an intermediate transfer belt 19 driven at a constant speed with the photosensitive drum 9 (hereinafter, toner from the photosensitive drum to the intermediate transfer belt). Image transfer is called belt transfer).
The belt transfer is performed by applying a predetermined bias voltage to the transfer bias roller 20a while the photosensitive drum 9 and the intermediate transfer belt 19 are in contact with each other. The intermediate transfer belt 19 includes Bk, which are sequentially formed on the photosensitive drum 9,
C, M, Y toner images are sequentially aligned on the same surface,
A belt transfer image of four-color superposition is formed, and thereafter, collective transfer is performed on transfer paper. This intermediate transfer belt unit will be described later.

On the photosensitive drum 9 side, the process proceeds to the step C after the step Bk. At a predetermined timing, the reading of the C image data by the color scanner 1 starts, and the laser beam writing based on the image data forms the C latent image. I do. The C developing device 15 starts to rotate the C developing sleeve 15a with respect to the developing position after the rear end of the previous Bk latent image has passed and before the leading end of the C latent image has arrived, so that the developer Then, the C latent image is developed with the C toner. Thereafter, the development of the C latent image area is continued, and when the rear end of the latent image has passed, the B
The developer on the C developing sleeve 15a is cut off in the same manner as in the case of the k developing device, and the developing is disabled. This is also completed before the leading end of the next M latent image arrives. The operations of reading image data, forming a latent image, and developing the M and Y processes are the same as those of the above-described Bk and C processes, and thus description thereof is omitted.

Next, the intermediate transfer belt unit will be described. The intermediate transfer belt 19 is stretched around a drive roller 21, a transfer bias roller 20a, a ground roller 20b, and a group of driven rollers, and is driven and controlled by a drive motor (not shown). Belt cleaning unit 2
Reference numeral 2 denotes a brush roller 22a, a rubber blade 22b, a mechanism 22c for contacting and separating from a belt, and the like.
After the Bk image of the color is transferred to the belt, the second, third and fourth colors are separated from the belt surface by the contact / separation mechanism 22c during the belt transfer. Then, the intermediate transfer belt 1
The images of the four colors are transferred one after another by accurately aligning the images of the four colors on the intermediate transfer belt 9, and a superimposed image of the four colors is formed on the intermediate transfer belt 19.

The paper transfer unit 23 includes a paper transfer bias roller 23a, a roller cleaning blade 23b, and a mechanism 23c for contacting and separating from the belt. The bias roller 23a is normally separated from the surface of the belt 19, but when transferring the four-color superimposed image formed on the surface of the intermediate transfer belt 19 onto the transfer paper at a time, the contact / separation mechanism 23c is timed. , And a predetermined bias voltage is applied to the roller 23a to perform transfer to paper. As shown in FIG. 1, the transfer paper 24 is fed by the paper feed roller 25 and the registration roller 26 at the timing when the leading end of the four-color superimposed image on the intermediate transfer belt surface reaches the paper transfer position. Is done.

As a method of driving the intermediate transfer belt 19, the following three methods are considered as operation methods after the belt transfer of the Bk toner image of the first color is completed to the end, and one of these methods is used. The intermediate transfer belt 19 is driven by efficiently combining a plurality of methods according to the copy size in terms of copy speed.

(1) Constant speed forward movement method After the Bk toner image is transferred to the belt, the forward movement is continued at a constant speed. Then, when the front end position of the Bk image on the surface of the intermediate transfer belt 19 reaches the belt transfer position at the contact portion with the photosensitive drum 9 again, the front end of the next C toner image is just on the photosensitive drum 9 side. The image is formed at appropriate timing so as to come to the position. As a result, the C image is belt-transferred on the intermediate transfer belt 19 while being accurately aligned with the Bk image. Thereafter, the process proceeds to the M and Y image processes by the same operation, and a belt transfer image of four-color superposition is obtained. Following the belt transfer process for the fourth color Y toner image,
While moving forward, the four-color superimposed toner image on the surface of the intermediate transfer belt 19 is collectively transferred to the transfer paper 24 as described above.

(2) Skip Forward Movement After the belt transfer of the Bk toner image has been completed, the intermediate transfer belt 19 is separated from the surface of the photosensitive drum 9 and skipped at a high speed in the forward movement direction, and the predetermined amount is moved. To the forward speed. After that, the photosensitive drum 9 is again
Is brought into contact with the intermediate transfer belt 19. Then, when the leading end position of the Bk image on the surface of the intermediate transfer belt 19 reaches the belt transfer position again, the photosensitive drum 9 side sets the timing so that the leading end of the next C toner image is exactly at that position. Is formed. As a result, the C image is transferred to the Bk image while being accurately aligned with and superimposed on the Bk image. Thereafter, the process proceeds to the M and Y image processes by the same operation, and a belt transfer image of four-color superposition is obtained. Subsequent to the fourth color Y toner image belt transfer step, the four-color superimposed toner image on the surface of the intermediate transfer belt 19 is collectively transferred onto the transfer paper 24 at the same forward movement speed.

(3) Reciprocating (Quick Return) Method When the belt transfer of the Bk toner image is completed, the intermediate transfer belt 19 is separated from the photosensitive drum 9 surface, and
At the same time, the forward movement is stopped and a high-speed return is made in the reverse direction. In this return, the Bk image tip position on the surface of the intermediate transfer belt 19 passes through a position equivalent to the belt transfer in the reverse direction, and after moving a predetermined distance, stops and enters a standby state. Next, when the leading end of the C toner image on the photosensitive drum 9 reaches a predetermined position before the belt transfer position, the intermediate transfer belt 19 is restarted in the forward movement direction. Further, the intermediate transfer belt 19 is brought into contact with the surface of the photosensitive drum 9 again. Also in this case, the C image is transferred to the intermediate transfer belt 19 under the condition that the C image accurately overlaps the Bk image on the intermediate transfer belt 19. After that, the same operation is performed to proceed to the Y and M image processes to obtain a belt transfer image of four-color superposition. Following the belt transfer process for the fourth color Y toner image,
The toner image moves forward at the same speed without returning, and the four-color superimposed toner image on the surface of the intermediate transfer belt 19 is collectively transferred to the transfer paper 24.

The transfer paper 24 on which the four-color superimposed toner image has been collectively transferred from the surface of the intermediate transfer belt 19 is conveyed to a fixing device 28 by a paper conveying unit 27, and is heated by a fixing roller 28a controlled to a predetermined temperature. The toner image is fused and fixed by the pressure roller 28b and is carried out to the copy tray 29 to obtain a full-color copy.

The photosensitive drum 9 after the belt transfer is
As shown in FIG. 2, the surface is cleaned by the photoconductor cleaning unit 10 (the pre-cleaning static eliminator 10a, the brush roller 10b, and the rubber blade 10c), and the static elimination lamp 11 uniformly removes the static electricity. After the transfer of the toner image onto the transfer paper, the surface of the intermediate transfer belt 19 is cleaned by pressing the cleaning unit 22 again by the contact / separation mechanism 22c.

At the time of repeat copying, the color scanner 1
Operation and image formation on the photosensitive drum 9 are performed on the first Y
(Fourth color) After the image process, 2
The process proceeds to the Bk (first color) image process for the sheet. In the intermediate transfer belt 19, the second Bk toner image is applied to the area where the surface is cleaned by the cleaning unit 22, following the batch transfer process of the first four-color superimposed image onto the transfer paper. The belt is transferred. After that, the operation is the same as that of the first sheet.

Incidentally, the paper feed cassettes 30, 31, 3 in FIG.
Reference numerals 2 and 33 denote transfer rollers of various sizes, and the registration rollers 2 are set in a timely manner from a cassette that stores the size paper designated by an operation panel (not shown).
Paper is fed and transported in six directions. Reference numeral 34 denotes a manual paper feed tray for OHP paper and thick paper.

The above is the description of the copy mode for obtaining four full colors. However, in the case of the three-color copy mode and the two-color copy mode, the same operation as described above is performed for the designated color and the number of times. Will be. Further, in the case of the single-color copy mode, until the predetermined number of sheets is completed, only the developing device of that color is set in the developing operation (spring), and the intermediate transfer belt 19 contacts the surface of the photosensitive drum 9. The copying operation is performed while the belt cleaner 22 is kept in contact with the intermediate transfer belt 19 in the forward movement direction at a constant speed.

The toner used in at least the developing units 15, 16 and 17 of the copying machine according to this embodiment is the same as the toners of the respective colors of cyan, magenta and yellow. It is composed of toner mixed with toner. Such toner has, for example, a particle size distribution as shown in FIG.
In addition, a toner having a charge amount distribution as shown in FIG. 4 is used.

That is, each of the developing units 15, 16, 1
As shown in FIG. 3, the toner of each color filled in No. 7 has an average particle size of about 7.5 μm in volume% as shown in FIG. Also, each of these toners
Two kinds of toners of the same color, that is, a light-colored toner Ta having a high light transmittance as shown by a solid line in FIG. 4 and a dark-colored toner Tb having a low light transmittance as shown by a broken line in FIG. It is composed of Further, the charge amount (Q / M) a of the light-colored toner Ta having a high light transmittance is different from that of the dark-colored toner Tb having a low light transmittance. It has the characteristic of being lower than the charge amount (Q / M) b. Here, the charge amount (Q / M) a of the light-colored toner Ta having a high light transmittance is (Q / M).
M) a ≒ −20 μC / g, the charge amount (Q / M) b of the dark toner Tb having a low light transmittance is (Q / M) b ≒ −30.
Each is set to μC / g. Therefore, in the copying machine according to the present embodiment, since the negatively charged toner is used as the toner, the dark color toner Tb having a large absolute value of the charge amount and a low light transmittance has a higher charge amount. Is set.

As shown in FIG. 5, the toner of each color obtained by mixing the two types of toners Ta and Tb of the same color having different light transmittance and charge amount has a distinct charge as shown in FIG. Although no peak in the charge amount appears, the charge amount distribution becomes broad.

On the other hand, in the developing process, the photosensitive drum 9
The amount of toner adhering (developing) on the upper side is determined by the gap between the photosensitive drum 9 and the developing rollers 15a, 16a, and 17a of each developing unit in a region where the charge of the latent image is not saturated (highlighted portion). Is approximately proportional to the electric field, and is in inverse proportion to the toner charge amount in a proper state in which the toner charge amount does not shift to the opposite polarity due to the above shift. Further, the two types of toners Ta and Tb in a single state before being mixed have characteristics as shown by the solid line in FIG. Accordingly, when the latent image is visualized using a toner in which these two types of toners Ta and Tb are mixed, first, as the electric field strength between the gaps increases, the charge amount first increases. The light toner Ta having a low light transmittance and a high light transmittance starts to adhere to the surface of the photosensitive drum 9, and the dark toner Tb having a high charge amount and a low light transmittance adheres to the surface of the photosensitive drum 9. As shown by a broken line in FIG. 6, both toners Ta and Tb are used as a combined toner image having a wide dynamic range.
Adheres to the surface of

As described above, in the highlight portion where the electric field strength of the latent image is low, the light-colored toner Ta having a low charge amount and a high light transmittance predominantly adheres to the surface of the photosensitive drum 9. Become. This is because the insufficiently charged toner (that is, the toner with a low charge amount) adheres to the background portion (highlight portion) of the transfer paper 24 and causes background smearing of the image. It can be inferred that the light-colored toner Ta having a small amount and a high light transmittance adheres to the photosensitive drum 9 earlier than other toners.

FIGS. 7A, 7B and 7C are schematic diagrams showing an example of the behavior of the toners Ta and Tb mixed as described above during development. That is, for example, FIG.
As shown in FIG. 7B, if there is a charge of “−7” on the photosensitive drum 9, as shown in FIG. 7B, the charge amount is “−5” and a light color having a low light transmittance is high. Only the toner Ta adheres on the photosensitive drum 9. Here, a dark toner Tb having a high charge amount of “−10” and a low light transmittance is used.
Has a charge amount higher than the charge [−7] of the photosensitive drum 9, and thus does not adhere to the surface of the photosensitive drum 9. Therefore, as described above, the latent image in the highlight portion where the charge of the photosensitive drum 9 is low is developed only with the light-colored toner Ta having a low charge amount and a high light transmittance.
An image with high gradation can be obtained. Next, as shown in FIG. 7C, in a portion where the charge on the photosensitive drum 9 is "-15" and the document image density is high, both the toners Ta and T are used.
b adheres to the photosensitive drum 9 and a dark toner image is reproduced. FIG. 8 shows the relationship between the toner adhesion amount and the image density of these toners Ta and Tb. Also, in FIG.
Each toner Ta and Tb before mixing, and both toners Ta after mixing
+ Tb shows the relationship between the electric field strength and the image density.

As shown in FIG. 9, the toner Ta having a low charge amount starts to adhere to the surface of the photosensitive drum 9 when the electric field is around 1000 V / cm.
Since the toner is a light color toner having a high light transmittance, the gradient of the developed image density is suppressed to be low, and the gradation in the highlight portion is improved. The toner Tb having a high charge amount has an electric field of 2200 V / cm.
From the vicinity, it starts to adhere to the surface of the photosensitive drum 9. Therefore, the image density of the mixed toner Ta + Tb is expressed in a form in which the toner Ta having a low charge amount and the toner Tb having a high charge amount are combined, as indicated by a broken line in FIG. Therefore, in the copying machine according to the present embodiment, a wider dynamic range of the image density can be ensured and the gradation of the highlight portion can be improved as compared with the case where the conventional toner having only one charge amount is used. Can be done.

Next, a case will be described in which a plurality of toners having different lightness, chroma, and hue are mixed as the toner used in the developing units 15, 16, and 17. FIG. 10 shows the relationship between the toner adhesion amount and color reproducibility in a conventional multicolor image forming apparatus. As is clear from FIG. 10, the saturation of the image increases as the amount of toner attached increases, but when the amount of toner attached increases to some extent, the amount of attachment becomes saturated, and the saturation further increases. Is gone. Also, it can be seen that the change in chroma is large in a region where the amount of toner adhesion is small. For this reason, in the conventional electrophotographic multicolor image forming apparatus, it is difficult to stably adhere the toner to an area where the amount of toner adhered is small (highlight area). Poor performance and stability are one of the causes of inferiority to ordinary photographic images. In the copying machine according to the present embodiment, when an image was formed using a conventional toner, it was difficult to stably adhere the toner in a highlight portion where the toner adhesion amount was 0.1 mg / cm ² or less. .

Therefore, in the copying machine according to the present embodiment, the toner Tc having different color reproducibility (saturation) with respect to the toner adhesion amount as shown by a broken line in FIG. , And the amount of charge is lower than the saturation of the other toner, and the amount of charge is set lower than the amount of charge of the other toner). As a result, similarly to the case of the above-described mixed toner Ta + Tb, the toner Tc having a low chroma predominantly adheres in the low electric field portion. That is, since the charge amount of the toner Tc is low in the low electric field portion, the toner Tc can be stably adhered to a highlight region where the toner adherence amount is 0.1 mg / cm ² or less. Become
Good color reproducibility with low saturation can be achieved. Regarding the brightness and hue of the toner, it is possible to improve the color reproducibility in the highlight portion of the image by using the toner for each color obtained by mixing a plurality of toners having different characteristics according to the same principle as described above. it can.

[0042]

According to the first to third aspects of the present invention, each of the toners of at least three colors used in the developing means is formed by mixing two or more kinds of toners having different characteristics from each other. Therefore, the latent image corresponding to one color formed in the image forming means is formed into a toner image by a plurality of toners of the same color having different characteristics from each other. The latent image can be visualized with toner having characteristics closest to the image. As a result, there is an excellent effect that it is possible to improve the gradation in the highlight portion of the image by a simple and low-cost method.

In particular, according to the second aspect of the present invention, each of the toners of at least three colors used in the developing means is a toner in which a plurality of toners having different light transmittances are mixed. The higher the light transmittance of each toner, the lower the charge amount of the toner. As a result, the highlight portion of the latent image is visualized with a light-colored toner having a low charge amount and a high light transmittance, and the high-density portion of the latent image has a high charge amount and a light transmittance. Since a visible image is formed with a low-density color toner, there is an excellent effect that an image having a wide dynamic range can be formed without performing image transfer a plurality of times for each color.

According to the third aspect of the present invention, each of the toners of at least three colors used in the developing means is a toner in which a plurality of toners having different lightness, saturation, and hue are mixed, In addition, the higher the brightness, saturation, and hue of each mixed toner, the lower the charge amount of the toner. As a result, the highlight portion of the latent image is visualized with a toner having a low charge amount and high brightness, saturation, and hue, and the high density portion of the latent image has a high charge amount and a high brightness. , Saturation and hue, the image will be visualized with low color toner.
In addition, there is an excellent effect that an image having a wide color reproduction range and improved gradation in a highlight portion can be formed.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a copying machine that is a multicolor image forming apparatus according to an embodiment.

FIG. 2 is an enlarged view around a photosensitive drum and an intermediate belt of the copying machine.

FIG. 3 is a graph showing a particle size distribution of a toner used in the copying machine.

FIG. 4 is a graph showing a charge amount distribution of toner used in the copying machine.

FIG. 5 is a graph showing a charge amount distribution of a mixed toner used in the copying machine.

FIG. 6 is a graph showing the relationship between the electric field strength of the toner and the amount of toner attached.

FIG. 7 is a schematic view showing the behavior of the toner during development.

FIG. 8 is a graph showing the relationship between the toner adhesion amount of the toner and the image density.

FIG. 9 is a graph showing the relationship between the electric field strength of the toner and the image density.

FIG. 10 is a graph showing the relationship between the toner adhesion amount of the toner and color reproducibility.

[Explanation of symbols]

 8 Writing Optical Unit 9 Photoconductor Drum 14 Bk Developing Unit 15 C Developing Unit 16 M Developing Unit 17 Y Developing Unit 19 Intermediate Transfer Belt 24 Transfer Paper 28 Fixing Unit Ta, Tb, Tc Toner

Claims (3)

[Claims] 1. A latent image forming means for forming a latent image on an image carrier using image light modulated based on image information, and the latent image carrier formed by the latent image forming means. Developing means for converting the latent image on the body into a toner image of the latent image using at least three color toners of yellow, magenta and cyan; and a visible image on the image carrier by the developing means. Transfer means for transferring the converted toner image onto a transfer member; and transferring the toner image transferred onto the transfer member by the transfer member.
In the multi-color image forming apparatus having a fixing unit for fixing on the transfer body, the toner used in the developing unit is a mixture of two or more types of toners having different characteristics from each other. A multicolor image forming apparatus comprising: a toner; 2. The multi-color image forming apparatus according to claim 1, wherein each of the toners of each color comprises a toner in which a plurality of toners having different light transmittances are mixed. A multicolor image forming apparatus, characterized in that the higher the light transmittance of a toner, the lower the charge amount of the toner. 3. The multi-color image forming apparatus according to claim 1, wherein each of the toners of the respective colors comprises a toner in which a plurality of toners having different lightness, saturation, and hue are mixed,
A multicolor image forming apparatus is characterized in that the higher the brightness, saturation, and hue of each mixed toner, the lower the charge amount of the toner.